The present invention relates to an electric generator for use on the rotational shaft of a turbocharger which can be driven by the exhaust energy discharged by an internal combustion engine, and more particularly to such an electric generator having a rotor which is capable of withstanding high-speed rotation and is resistant to heat.
In recent years, turbochargers find widespread use on internal combustion engines. The turbochargers have an exhaust turbine which is rotated by the heat energy of the exhaust gas discharged from the internal combustion engine. A compressor directly coupled to the exhaust turbine is driven thereby to deliver compressed air to the internal combustion engine. One turbocharger design includes a generator/motor having a rotor mounted on the rotational shaft of the turbocharger. When the exhaust turbine is rotated by the exhaust gas, the rotor is also driven to generate electric power so that the exhaust energy can be regenerated as electric energy. The rotor can also be driven by the electric power supplied from an inverter powered by a battery to assist the compressor in supercharging the engine.
U.S. Pat. No. 4,433,261 discloses, as such a turbocharger incorporating such a directly coupled generator, a gas turbine generator having a rotor disposed between turbine and compressor blades and comprising divided permanent magnets fitted in a cylinder.
In the disclosed gas turbine generator, the permanent-magnet rotor is mounted on the turbine shaft which rotates at a high speed, thus providing an AC machine. However, the size and arrangement of the rotor are not suitable for high-speed rotation. More specifically, where the rotor is of a large diameter and has an increased weight, its inertia is large and its response to changes in the rotational speed thereof is poor. Further, the rotational shaft produces flexural vibration with respect to its bearings, and suffers the danger of breakage due to resonance. The rotor composed of divided permanent magnets fitted in a cylinder is not suitable for high-speed rotation since it is difficult to reduce the-amount of imbalance thereof.
Inasmuch as the rotational shaft and the permanent magnets are held in direct contact with each other, the heat from the turbine blades exposed to the high-temperature exhaust gas is transmitted through the rotational shaft to the permanent magnets. Therefore, the permanent magnets are demagnetized by an increase in temperature, resulting in a reduction in the amount of electric power generated by the generator.